There are many applications today where it is desired to use flat panel displays, typically liquid crystal (LC) flat-panel displays (LCDs). For example, the display may be a head-up display (HUD) employed in an aircraft, that allows a user to view multiple scenes and/or multiple types of data at the same time without moving his or her head to look at different individual displays. With a HUD the aircraft pilot can see the scene outside of his or her cockpit window and at the same time view a variety of flight data overlaid on the image of the external scene. The pilot receives both types of information at the same time, the outside scene and the flight data, without having to glance down into the cockpit to view various flight data instruments. This is a significant advantage and can substantially improve pilot performance and safety. Head-up displays can be used in many other applications. Another example is a projection display where a back-light is directed through a flat panel LC display and the resulting image projected onto a screen in the viewer's line of vision. This arrangement is often used where a large size image is desired to be displayed. Both of these examples often need powerful back-lights to illuminate the LC display panel so that the resulting image can be easily seen against an outdoor scene in a head-up display or when enlarged many times in a projection display being viewed in significant ambient light, or both.
FIG. 1 illustrates typical prior art head-up display 20 that includes lamp 22 providing light beam 23 that illuminates liquid crystal display (LCD) panel 24 of variable transmittance. The desired display information or data is provided via input 26 to display driver 28, wherein it is converted to the proper signal format and sent along link 29 to drive display panel 24. Such arrangements are conventional. Data image 25 in optical form is emitted by display panel 24. Image 25 from display panel 24 passes through optional lens 30, which desirably provides focusing. Focused image 31 is projected on combiner plate 32 where it is partially reflected toward user 34 as image 33. Image combiner 32 is usually a partially reflecting, partially transmitting (e.g., glass) plate that is in user's line of vision 35. The user looks through combiner plate 32 at, for example, image 37 of external scene 36 and at the same time is able to see the data image 31, 33 that is being reflected off combiner plate 32. Elements 30-37 form optical portion 38 of display 20. For convenience of explanation it is assumed herein that image 37 results from an external scene, but this is not essential and not intended to be limiting. Image 37 may originate from any type of source. In many cases image 37 of external (or other) scene 36 can vary widely in brightness. In these circumstances, it is desirable that data image 31, 33 also be adjustable over a wide brightness range. Otherwise it may not be visible against a bright external scene (or other image). In order to achieve a wide brightness range for data image 25, 31, 33 being generated by display panel 24, it is often necessary that lamp 22 used to illuminate display panel 24 be very bright, for example, an order of magnitude or more brighter than lamps commonly employed with prior art LCD displays. It has been found that when such very bright lamps are used, the properties of typical LCD panels change with lamp brightness and undesirable artifacts such as flicker or a retained image can occur.
FIG. 2 illustrates typical projection display 20′ that, other than optical portion 38′ is substantially the same as display 20 of FIG. 1. Like reference numbers are used to identify like elements. Thus, elements 22-25 are substantially the same in FIG. 2 as in FIG. 1 and the discussion thereof in connection with FIG. 1 is incorporated herein by reference. Reference numbers with a prime (′) mark are used to identify elements in optional portion 38′ of FIG. 2 that perform functions analogous to those of elements in optical portion 38 of FIG. 1. Elements 30′-35′ form optical portion 38′. In system 20′ of FIG. 2, optical image 31′ is projected by lens 30′ onto projection screen or plate 32′ that is located in line of sight 35′ of viewer 34′, so that viewer 34′ can easily see image 33′ from projection screen 32′. In this example, projection screen 32′ is semi-transparent or translucent so that image 31′, 33′ is visible to viewer 34′, but this is not essential. Viewer 34′ may also be located on the same side of plate 32′ as lens 30 and view the projected image by reflection. Either arrangement is useful. While the arrangement shown in FIG. 2 is useful and widely employed it suffers from a number of disadvantages well known in the art. For example, if there is significant ambient light around projection screen 32′, then image 33′ may be degraded or difficult to see. Further, where the area of image 31′ must be very large, there is a rapid decrease in the intensity of image 31′ seen by viewer 34′. For constant lamp brightness, the image intensity drops off approximately as the square of the image dimension. For example, doubling the image size reduces the brightness to about one-fourth of its original value. When projection displays have large screen size and/or must operate in significant ambient light or both, then very bright back-light lamps 22 are often used. It has been found that when such very bright lamps are used, the properties (e.g., flicker, retained image, etc.) of typical LCD display panels change with lamp brightness. When only a single brightness is needed, these artifacts can generally be compensated. But when variable brightness is needed, such prior art systems are unable to provide compensation over a range of brightness. This is undesirable. The brighter the lamp, the greater the need to provide a display system that adapts to back-light brightness variations.
Accordingly, it is desirable to provide an improved display that permits significant variations in brightness while compensating its output for such variations so as to maintain substantially optimized properties over such range of brightness. In addition, it is desirable that the compensation arrangement be electronic rather than mechanical so as to not cause a significant increase in weight or size of the display. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.